1979 Mercury 90HP Inline 6 One Cylinder Dead, Needs Rebuild. Please advise.

[elr]

Hi Folks,

I have a '80 Starcraft SS 16' (love the boat) with a '79 Mercury 90hp inline 6 (love the motor). I've had for maybe 15 years and made many happy memories with it.

​

Synopsis of problem:
In the spring of '17 we brought it out of storage aiming for the lake. Motor ran but not the best. We took it out and gave it a seafoam treatment thru the carbs trying to get it to clear up. It seemed like it wasn't running on all cylinders all the time. Gave up for time being and came home.

2nd time out same story except that it settled down to only running on 5 cylinders by the end. Came home and had a friend do a compression test... Don't remember the numbers except that top cylinder (#1) was dead. Pushed it back under a tarp and moped about it for the next two years.

Fast forward to now... I pulled it out, cleaned it up and did another compression test. This test was done cold because I couldn't get it running. I did spray fogging oil in all cylinders before and during compression test. Did each cylinder a couple times and following are averages in reverse order:

Cylinder #6 (bottom): 145
#5: 135
#4: 136
#3: 134
#2: 129
#1: 31 !!!

So pretty much confirmed what I knew. I was pleasantly surprised with 5 of the cylinders. Looking in the spark plug hole of #1 you can see scoring on the cylinder wall.

I'd like to rebuild it. I've never rebuild an engine but am mechanically inclined. (seeing as how I've recently started the 30-month apprenticeship route to becoming an A&P mechanic, I should be) So my questions...

#1. Does it even make sense to try it? I.e. Are parts still available for a 40-year old motor? If so, ballpark cost for most likely scenario?
#2. Also If so, how likely to need bored out and oversized? Or is there even much difference labor or parts-wise between that and just honing and new rings?
#3. How involved is it? Does pretty much everything need to come apart including all the bottom end stuff like connecting rods, crankshaft, etc.?
#4. Anyone willing to venture a guesstimate on # of hours required to bring her back to life?
#5. Why did it happen in the first place and what can I do to make sure it doesn't happen again?

I have received good help in the past from iboats for numerous odds and ends on this boat and would love to hear from folks like Chris1956, OldMercsRule, Gomer50, achris, and whoever else knows what they're talking about. Thanks.





serial number (on outside, can't find it on the engine block)

 

[elr]

Forgot to finish...

Serial number on outside of motor is 5221045. I couldn't find it on the engine block but pretty sure it is OEM. Anyone know where that number is on the block and should it be the same?

 

[GA_Boater]

Why did it happen? Using Seafoam and trying run the motor through the poor running instead of fixing the carbs is my guess. Probably starved two top 2 cylinders for oil and #1 let go before #2.

Pulling a piston means tearing the whole motor apart because it has no separate head and boring if needed, must be done by a shop specializing in blind boring.

 

[jimmbo]

# 1 is the first to suffer, when the engine is starved of cooling water. Parts are still available from Mercury. While I love the inline 6, you might be less than impressed at the price of an overhaul, or a rebuild

 

[Scott Danforth]

cheapest way to get an ole inline 6 running with a scored cylinder: scrap what you have, buy a complete and running inline motor

second cheapest is to scrap what you have and buy a V6

most expensive way is to rebuild what you have - seriously, look into machine shops that do blind bore machining.

 

[jimmbo]

The V6 is leaps and bounds ahead of the inline

 

[Faztbullet]

#1...

Does it even make sense to try it? .....No unless its for a restore
#2. Also If so, how likely to need bored out and oversized? 95% as the cylinder wears around the intake/exhaust ports
#3. How involved is it?...pretty involved and will need special ring compressers.
#4. Anyone willing to venture a guesstimate on # of hours.. if you have all the parts and machine work complete you can make it smoke in about 10 hrs
#5. Why did it happen in the first place....fuel,cooling trigger if not a ADI​

 

[elr]

Thanks for replies... I kinda figured it was from trying to run it probably without some cylinders getting proper fuel/lubrication.

I keep getting people talking about sleeves... Can someone tell me if these cylinders are sleeved or not? And call me ignorant (because I am), but what is blind boring?

Jimmbo and Scott Danforth, is the expense in the labor of complete overhaul or the parts? Just figuring parts and labor to bore are we talking about $500 or 1k or what?

 

[GA_Boater]

This video might explain blind boring. When the camera looks down the cylinder, the spark plug is visible because the block and what would be the head is one piece, not two. So boring must be precise so the block isn't bored into the combustion chamber. The video isn't the actual boring process, it's the preliminary measurement using CNC equipment.

 

[GA_Boater]

Price sheet for blind boring - 80 bucks a hole for an inline 6;

http://precisioncycleinc.com/servic...oring-services/cylinder-boring-pricing-sheet/

 

[QBhoy]

Oh it’s a crying shame and even hate the words I’m putting down here...but after a similar issue with another such legend and classic engine (Yamaha V4) as your straight 6 lovely machine...I’m afraid I ended up facing reality and making sense of it eventually...and buying a good used 2.0 V6 merc instead. Like your engine. The cost of parts to rebuild are about or more as the cost of buying a good used V6 merc it even a good used boat, trailer and V6 merc attached to it...as I did.
Youll get strong money just selling the engine for parts as I did. Even the gearbox will be worth half what your engine would be. Never mind the power trim unit.
Best advice I can give you.

 

[Faztbullet]

Just figuring parts and labor to bore are we talking about $500 or 1k or what? ​

Boring...80x6=$480
Pistons...80x6=$480
Gasket kit....$88
carb kit...20x3=$60
impeller kit....$40
upper/lower bearing...$35
rod bearing......15x6=$90
wrist pin brg...9x6=$54
Total..... $1327
Look at Craiglist and other listings like Facebook local to find a different engine. Here is one from my area that would be a 14 year upgrade.
https://nashville.craigslist.org/bpo/d/ashland-city-mercury-outboard/6922507090.html

 

[elr]

Thanks Faztbullet, for the great parts breakdown.

Maybe my ignorance popping up again, but I was imagining everything x1, not x6... Why do all 6 holes need to be bored and 6 new pistons installed? Engine out of balance? Or is it just best practice to do as a matched set?

Also same for upper/lower bearings, rod bearings, wrist pin bearings, etc... replace just to be on safe side or absolute necessity whenever rebuilding an engine?

 

[elr]

I was working on a list as well and mine was more like...

(1) Boring...80
(1) Oversize Piston w/rings and wrist pin... 130
Gasket Set... 72
Carb kit... 60
Impellor.... 40 (these last numbers taken from faztbullet)
Incidentals... 118

Total.... $500

I would pay plus my time to have my old motor back... What am I missing?

 

[GA_Boater]

Well, the scuffed aluminum didn't all go out the exhaust. Some was circulating through the motor. Just doing one cylinder might work for a while. And then again..................

 

[GA_Boater]

Well, the scuffed aluminum didn't all go out the exhaust. Some was circulating through the motor. Just doing one cylinder might work for a while. And then again..................

I would whatever is reasonable to keep a TOP alive, but there comes a time.

 

[flyingscott]

I ran a 78 ss 160 and it had an 85 hp max rating on it. Don't listen to anyone who tells you to put a V6 on that boat unless you have a death wish. A V4 OMC 85 hp would be a good choice. A newer 3 cylinder 75 hp merc is an excellent choice. I ran an 1986 OMC 3 cylinder 70 hp and it was a great match for that boat.

 

[elr]

Yes, my plate says 85 max as well, but it came from the dealer with a 90 strapped on it. I’ll admit it’s a little tail heavy, but fine by keeping passengers or other weight up front. I have a very tentative 70hp that may be an option but was afraid it would be sluggish. I loved the 90 when it worked.... wakeboard, skiing one or two, (even barefoot,) kneeboarding and tubing we did it all.... then cruise down the lake at 45-46mph lightly loaded. Has been a fun boat. I’m not sentimental at all, am I?

 

[Scott Danforth]

I was working on a list as well and mine was more like...

(1) Boring...80
(1) Oversize Piston w/rings and wrist pin... 130
Gasket Set... 72
Carb kit... 60
Impellor.... 40 (these last numbers taken from faztbullet)
Incidentals... 118

Total.... $500

I would pay plus my time to have my old motor back... What am I missing?

what you are missing is most machine shops wont just bore one cylinder if the others so much as have a scratch unless you sign a waiver.

 

[elr]

Ok, at this point I'm proceeding with the tear down and will see what I find. I do have a local machine shop that may be able to do it for me.

My question is the power head removal. My Clymer manual makes it sound so simple... once all wiring, etc is off and 6 nuts that are holding the power head to drive shaft housing are off it says..."Lift power head from drive shaft housing and install on stand." Really...? Does the crank shaft just slide out from the lower drive shaft housing?